The present invention relates to a method and filling station for introducing a compressed gas stream into one or more vessels with a controlled filling temperature. More particularly, the present invention relates to such a method and filling station in which a vapor phase stream composed of a vapor phase of a cryogen stored within a cryogenic, liquid storage tank is compressed to form the compressed gas stream that is introduced into the vessel(s). Even more particularly, the present invention relates to such a method and filling station in which a liquid phase of the cryogen is heated to maintain the vapor phase at a constant density and a filling temperature of the compressed gas stream is controlled by selective addition of heat thereto.
In filling various vessels with compressed gases within set time intervals, the temperature of the compressed gas within the vessel will tend to rise due to the inability of the vessel to dissipate the heat produced from the enthalpy of the incoming compressed gas. Such vessels can be compressed gas cylinders that are used to store industrial gases and vehicle fuel tanks in which a fuel in the form of compressed natural gas or hydrogen is to be stored.
In all such filling applications, it is important to control the temperature of the compressed gas within the vessel being filled. An important reason for such temperature control is to ensure that the vessel is filled to capacity. For instance, if the temperature within the vessel being filled is allowed to rise, the pressure of the compressed gas within the vessel will eventually drop to a pressure in equilibrium with ambient temperature (the settle pressure) as heat dissipates from the vessel. Thus, the pressure reached within the vessel during filling will not guarantee that the vessel will be filled to the desired capacity. As a result, either the vessel has to be topped off after having been initially filled, or the filling pressure has to be raised substantially in excess of the settled pressure, or the filling has to be accomplished at a rate slow enough so that near ambient temperature conditions are maintained within the vessel during vessel filling.
As may be appreciated, such a filling procedure is time consuming and therefore not very desirable in industrial filling applications. It is particularly undesirable in vehicle fuel tank filling applications in that consumers have an expectation of filling times with respect to alternative fuels, such as compressed natural gas and hydrogen, that are comparable to those experienced with conventional petroleum based fuels. Preferably, a vehicle fuel tank should be able to be completely filled from an empty condition with compressed natural gas or hydrogen in between about three and about five minutes.
When very short filling times are contemplated, such as time periods that have been discussed above, another problem surfaces. The time period is so short that the heat generated during filling can exceed the upper structural limiting temperature of the tank being filled. Additionally, when the compressed gas such as hydrogen is to be stored as a liquid, the resultant filling temperature can be below the lower structural limiting temperatures of the fuel tank. At such temperatures, failure occurs due to materials becoming brittle. These problems can be particularly exacerbated in fuel tanks that are designed to store hydrogen. In order for the vehicle to have sufficient range, hydrogen gas must be stored at anywhere from between about 5,000 psi and about 10,000 psi. A fuel tank designed to hold such pressure, if conventionally fabricated, would be quite heavy. Therefore, such fuel tanks are and will be fabricated from lighter materials such as carbon fiber and resin. The tank mass to gas ratio in carbon fiber reinforced tanks is much lower than the ratio for conventional tanks. Consequently, these lighter tanks are more prone to encounter high temperatures during filling.
The problem of filling pressure vessels to a desired pressure with reasonable time periods has been dealt with in at least one patent, namely, U.S. Pat. No. 5,934,081. In such patent, in order to ensure that compressed gas cylinders are completely filled, without being topped off, the filling temperature of the compressed gas is controlled. In this patent, the gas is stored within a cryogenic liquid storage tank. A liquid stream, composed of the liquid phase, is pumped and then vaporized within a vaporizer. Pressure within the tank is maintained by a pressurized stream of pumped liquid that is vaporized and fed to a head space of the liquid storage tank and into the vapor phase. Another stream of liquid, pressurized by the pump, is left unvaporized and introduced into the vaporized stream from the vaporizer to cool the compressed gas being introduced into the cylinders by direct heat exchange.
The problem with the foregoing patent is that the liquid storage tank must be located above the pump in order to allow gravity feeding to the pump. In order to allow for the underground storage of liquid, in U.S. Pat. No. 5,787,940, which concerns the use of liquefied natural gas as a fuel source, an underground storage tank is disclosed that also has an associated sump and a submerged pump to pump a stream of liquefied natural gas above ground to a liquefied natural gas fueled vehicle. The advantage of the sump is to allow for pump maintenance through the sump. In any case, the pumped liquid has to be vaporized and therefore, such a system is rather complex and difficult to control.
The problem of filling vehicle fuel tanks with hydrogen has been recognized in U.S. Pat. No. 6,432,283. In this patent, a hydrogen fueled replenishment system is disclosed in which hydrogen is generated onsite by an electrolytic cell. The hydrogen is compressed to a filling pressure and in order to compensate for the resultant temperature rise, the fill rate is adjusted to accomplish delivery to the vehicle within a minimum of time to meet the filling requirements of the vehicle. In another system disclosed in U.S. Pat. No. 5,628,349 that involves the dispensing of a pressurized gas, for instance, ompressed natural gas from a pressurized gas source, the temperature within the receiving tank is monitored and is used by a computer to adjust the fill pressure to compensate for the temperature and pressure rise occurring within the compressed gas during the filling of the tank. The problem with both of these systems is that they would have limited utility with respect to lightweight, carbon fiber fuel tanks for use in hydrogen fueled vehicles due to the higher pressures involved and the relatively short fill time periods.
As can be appreciated from the above discussion, an advantageous filling station would utilize an underground storage tank in which the compressed gas to be dispensed were stored as a liquid and gas was compressed from the vapor phase. The problem with such operation is that as gas is taken from the vapor phase, the pressure decreases within the storage tank. This results in inefficient compressor operation. In U.S. Pat. No. 5,520,000 a system is disclosed that is designed to deliver a gas, such as hydrogen for a hydrogen fuel tank at a temperature of 6,000 psia in a manner that maximizes the compressor output. This is accomplished by using a gas liquid mixer before the compressor or compressor stages in which gas and liquid are mixed in a packing contained within a miniature column to control the temperature of the gas entering the compressor and thereby maximize the output rate of the compressor. The temperature of the gas that is introduced into a storage bank is controlled by either heating or cooling the gas in a subsequent heat exchanger.
U.S. Pat. No. 5,243,821 discloses a method of delivering a high pressure gas in which a piston-type pump/compressor is adapted to pump liquid, vaporized liquid or a two-phase mixture of vapor and liquid while maintaining the inlet fluid under cryogenic conditions. The gas/liquid composition of the input to the pump/compressor is varied to control the mass flow rate to enable a variable gas outlet feed. The gas can be hydrogen. Flow control is achieved by varying the input density of fluids to the pump. When the pump compressor is drawing vapor only, a conventional auxiliary pressure building circuit is employed to maintain pressure within a liquid storage vessel. In situations in which the pump/compressor is providing more of a compressing function, leakage from piston rings used in the pump/compressor, is recirculated back to the liquid within the storage tank to result in the minimum possible pressure rise in the tank. This leakage, referred to in the patent as xe2x80x9cblow-byxe2x80x9d is first cooled in an aftercooler to recover refrigeration before being recirculated back to the tank to preserve the coldest recycle temperature possible. Blow-by may also be returned to vapor space of the tank under extremely high flow conditions. The output of this system is heated within vaporizers o produce a gaseous product at about ambient conditions. There is no disclosure in this patent of controlling the density of the gas being compressed by such blow-by nor is there any attempt to control a filling temperature of storage tanks to be filled.
As will be discussed, the present invention provides a vessel filling method and filling station in which the filling temperature of the compressed gas being introduced into the vessel can be accurately controlled by apparatus that is far less complex than the prior art to allow a vessel to be very rapidly and accurately filled with a desired mass of compressed gas and also, without exceeding either the upper or the lower structural temperature limit of the vessel being filled.
The present invention provides a method of introducing a compressed gas stream into at least one vessel with a controlled fill temperature. In accordance with the invention, a cryogen is stored in a cryogenic, liquid storage tank in a vapor phase and liquid phase. A vapor phase stream is compressed to produce a compressed gas stream. Heat is selectively added to the liquid phase such that a pressure within the vapor phase is held constant and therefore the vapor phase is maintained at constant density. At least part of the compressed gas stream is introduced into the at least one vessel. The fill temperature of the compressed-gas stream is controlled upon introduction thereof into the at least one vessel to be at least about equal to the controlled filling temperature by selectively adding further heat to at least one of the vapor phase stream prior to compression and the compressed gas stream.
By holding the density of the vapor phase constant, the output conditions after compression are also held substantially constant to allow the fill temperature of the compressed gas stream to be accurately and practically controlled even when the fill times are very rapid, for instance, from about 3 minutes to about 5 minutes. The control of density contemplated by the present invention can be accomplished with the most basic of control logic, that is, pressure control, and hence, does not depend on such complicated systems of the prior art involving mixing columns and multiple temperature transducer inputs. Furthermore, by compressing a stream of the vapor phase, although storage tank placement can be above ground, it does not have to be and can be located below ground to reduce the footprint of a filling station incorporating such method. As stated previously, the control of the fill temperature is important so as to prevent underfilling vessels and also to prevent temperature limitations of materials used in fabricating such storage vessels to be exceeded. As will be discussed, the fill temperature can be controlled so that the compressed gas temperature within the vessel during filling is maintained near ambient temperature of the surroundings of the vessel to guarantee the foregoing intended filling of the vessel and temperature limitations.
The heat can be added to the liquid phase by selectively introducing a subsidiary stream, composed of a remaining part of the compressed gas stream, into the liquid phase. First and second subsidiary streams can be formed from the remaining part of said compressed gas stream. In such case, the subsidiary stream introduced into the liquid phase is the first subsidiary stream and the second subsidiary stream is selectively introduced into an inlet of a compressor used in compressing the vapor phase stream to add the further heat to the vapor phase stream In alternative embodiments, further heat is added to the compressed gas stream with either a thermal ballast or a trim heater and flow of a by-pass stream, by-passing said trim heater. Flow of the by-pass stream is controlled to selectively control the further heat added to the compressed gas stream and therefore, the fill temperature. In case of a thermal ballast, the thermal ballast is heated to at least partially store the further heat prior to introduction of the least part of the compressed gas stream into the vessel.
The vapor phase stream can be compressed in a compressor. Prior to the introduction of the compressed gas stream into the at least one vessel, the compressed gas stream is recirculated from the compressor to the liquid phase until the compressed gas temperature at an outlet of the compressor is equal to a compressor delivery set point temperature. After the compressed gas temperature has reached the compressor delivery set point temperature, the at least part of the compressed gas stream is introduced into the at least one vessel and flow of the subsidiary stream to be introduced into the liquid phase is adjusted so that said pressure is held constant.
In such embodiments, the compressed gas can be hydrogen and the at least one vessel is a vehicle fuel tank. The vehicle fuel tank in such case can be formed of a carbon fiber material. The time period to completely fill said vehicle fuel tank can be between about 3 and about 5 minutes and the pressure within the vehicle fuel tank when completely filled can be between about 5,000 psi and about 10,000 psi.
In another aspect, the present invention provides a filling station for introducing a compressed gas stream into at least one vessel with a controlled fill temperature. In accordance with this aspect of the present invention, the filling station comprises a cryogenic, liquid storage tank for storing a cryogen as a vapor phase located above a liquid phase. A compressor for compressing a vapor phase stream of the vapor phase is provided to produce a compressed gas stream. An outlet conduit connected to the compressor introduces at least part of said compressed gas stream into the at least one vessel. A recirculation conduit is connected between the storage tank and an outlet of the compressor such that a subsidiary stream composed of at least a portion of a remaining part of the compressed gas stream, is able to be introduced into the liquid phase, thereby to add heat to said liquid phase and to pressurize the vapor phase. A valve is provided to control flow within said recirculation conduit and a pressure sensor senses pressure within the vapor phase. A controller responsive to the sensed pressure controls the valve such that the pressure is constant and therefore said density of the vapor phase is also held constant. A trim heater adds further heat to the compressed gas stream and a by-pass line by-passes the trim heater. A diverter valve controls the portion of the compressed gas stream that flows through the by-pass line and therefore, the filling temperature to be at least about equal to the controlled filling temperature.
In alternative embodiment of the filling station, first and second recirculation conduits are connected to an outlet of the compressor. The first recirculation conduit is connected to said storage tank such that a first subsidiary stream composed of at least a portion of a remaining part of the compressed gas stream is able to be introduced into said liquid phase to add heat to the liquid phase and thereby pressurize the vapor phase. The second recirculation conduit is connected to an inlet of the compressor to recirculate a second subsidiary stream composed of another portion of a remaining part of the compressed gas stream to an inlet of the compressor so that further heat is added to the vapor phase stream. A first valve controls flow within the first recirculation conduit and a pressure sensor is provided to sense pressure within the vapor phase. A controller responsive to the sensed pressure controls the first valve such that the pressure is constant and therefore the density of the vapor phase is also held constant. A second valve controls flow within the second conduit and therefore a filling temperature of the compressed gas stream to be at least about equal to the controlled fill temperature.
In either embodiment of the filling station, the compressed gas can be hydrogen and the at least one vessel can be a vehicle fuel tank. The storage tank can be located underground and beneath the compressor.